Documentation example code invalid for latest library version

[jmal0]

From https://s2geometry.io/devguide/s2shapeindex.html:

void TestContainment(const vector<S2Point>& points,
                     const vector<S2Polygon*>& polygons) {
  MutableS2ShapeIndex index;
  for (auto polygon : polygons) {
    index.Add(absl::make_unique<S2Polygon::Shape>(polygon));
  }
  auto query = MakeS2ContainsPointQuery(&index);
  for (const auto& point: points) {
    for (S2Shape* shape : query.GetContainingShapes(point)) {
      S2Polygon* polygon = polygons[shape->id()];
      ... do something with (point, polygon) ...
    }
  }
}

This code does not compile for several reasons:

• GetContainingShapes now returns std::vector<const S2Shape*>
• There is no id() function in S2Shape (removed in this commit: 84bfd2c)